1. Field of the Invention
The present invention relates to image processing techniques for processing an image including a latent image area and a background area, and pertains to the field of preventing, detecting, and suppressing illegal copying of printed material. More specifically, the present invention relates to a technique for producing printed material which can be distinguished from its copies.
2. Description of the Related Art
In order to suppress illegal copying of important documents such as a resident card, a certified copy of family register, etc. and distinguish originals from copies, specially printed paper called security paper is commonly used.
A sheet of security paper generally has a warning message such as “void copy” printed at a density such that the message can be reproduced with a copy machine and a background at a density such that the background cannot be easily reproduced with a copy machine. The densities of the warning message and the background excluding the warning message are set such that they cannot be distinguished from each other by human eyes. Hereafter, an area which is difficult to reproduce with a copy machine is called a latent image (or a latent image area), an area which is reproducible with a copy machine is called a background (or a background area), and the area including both the latent image and the background is called a ground image. When printed material on which the ground image is recorded is copied, the warning message is clearly shown on the copy.
However, in the case in which the security paper which is specially treated is used as described above, there is a cost problem since the security paper must be prepared in advance. In order to solve this problem, techniques for generating the ground image by software without using the security paper have been proposed. For example, according to Japanese Patent Laid-Open No. 2001-238075, a ground image including a dither pattern whose density is high relative to the resolution of a copy machine which reads the ground image and a dither pattern whose density is low relative to the resolution of the copy machine is generated, and an image obtained by superimposing the generated ground image and output data such as a document image is output.
In both of the cases in which the security paper is used and the ground image is generated by software, a warning message is shown when a copy is made, so that people can determine whether the printed material is a copy. However, systems, machinery and the like cannot determine whether such printed material is a copy.
In addition, although copying can be potentially suppressed by the warning message, it is difficult to essentially prevent copying, and it is also difficult to determine the printer used for outputting printed material, the date and time at which the printed material was outputted, and the user who outputted the printed material.